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SubjectRe: How how latent should non-preemptive scheduling be?

On Mon, 22 Sep 2008, Ingo Molnar wrote:
> * Sitsofe Wheeler <sitsofe@yahoo.com> wrote:
> >>
> >> sounds like potential SMM triggered latencies.
> >
> > I have just gone away and read about the SMM (
> > http://blogs.msdn.com/carmencr/archive/2005/08/31/458609.aspx ). If
> > you're right there is pretty much nothing that can be done about the
> > problem : (
>
> well, since they went away after you enabled CONFIG_PREEMPT=y, they are
> definitely in-kernel latencies, not any external SMM latencies.
>
> I.e. they are inherently fixable. Could you enable:
>
> CONFIG_DYNAMIC_FTRACE=y
> CONFIG_FTRACE_MCOUNT_RECORD=y
>
> that should make the traces a lot more verbose - every kernel function
> executed in the latency path will be logged. That way we'll be able to
> say which one takes that long.
>
> note, you might have to increase /debug/tracing/trace_entries to get a
> long enough trace to capture the relevant portion of the latency.

Also note, to modify trace_entries, you must be in the none (nop?) tracer,
otherwise the size will not be effected.

If you find the trace is also too big, you can echo a list of functions
into:

/debug/tracing/set_ftrace_notrace

to not trace those functions. using '>' will remove any existing function
in that file, but using '>>' will append functions to the file.

For a list of functions that you can add, see:

/debug/tracing/available_filter_functions

-- Steve



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